The use of a microwave based system with a single antenna and radiometer to detect thermal anomalies in the breast has been investigated in several clinical studies. We are now developing a multiple antenna system, based upon the results of these initial studies, to provide greater resolution and sensitivity with a dramatic reduction in examination time. In this study characterization procedures that define spatial and depth coverage parameters within phantom models will be used to design the antenna array for optimal performance. Improved performance is achievable with this system since antenna elements can now be site-optimized. Rapid data- acquisition eliminates environmental fluctuations due to patient or equipment. By mounting the antenna array on contoured plates, compression techniques can be used to provide views similar to those obtained with mammography. Reduced tissue thickness together with viewing from opposing surfaces will overcome problems associated with deep lesions and large breasts. In addition to performance improvements, the use of multiple antennas in an array configuration permits rapid data acquisition through electronic scanning and thermal matching at all sites simultaneously. Computerized data processing provides easily interpretable, graphic results available for display immediately after examination.